Hidden in Sand does not work properly

[Zenzei]

My experience with iron dragons is limited, only used them once or twice in single player, but I haven't been impressed by them so far. They were mostly just very expensive meat shield and they sucked at trampling.

So yeah, I think Hidden in Sand is better than 3 iron dragons. Also, HiS is only sixth level spell so you can use it long before any iron dragons.